As applications of superconductivity such as MRI and SQUID develop, there is an urgent need for cryogenic refrigerators which achieve very low temperatures of from a few K. to some tens K. more facilely and more stably. Helium gas is usually used as a refrigerant in such refrigerators, which is repeatedly compressed and expanded to make a cooled section. Heat is pumped up from the cooled section to the hot section by means of a heat accumulator or a heat exchanger. Since the refrigerator using the heat accumulator is relatively simple in its structure, it is suitably used as a compact refrigerator installed in apparatuses. Typical examples of this type of refrigerator are the Stirling refrigerator and the Gifford-McMahon refrigerator.
Heat reserving materials which have a large specific heat capacity and a good heat conductivity at the working temperature are desired for use in the refrigerator of the heat accumulating type. Copper, lead and alloys thereof are conventionally used for this purpose since the specific heat thereof does not drop until relatively low temperatures while the heat conductivity thereof is good.
Heat capacity of these metals or alloys, however, results from the lattice vibration and accordingly the specific heat thereof rapidly drops as the temperature is lowered to 10-20 K. Therefore, it was difficult to achieve a very low temperature lower than 20 K., especially lower than 10 K. by using a refrigerator in which these metals or alloys were used as a heat reserving material. Very low temperatures of a few K., e.g. the liquid helium temperature (4.2 K. under atmospheric pressure) could not be achieved by these refrigerators.
Magnetic substances having an anomalous specific heat caused by magnetic transition were proposed as heat reserving materials in place of the conventionally used copper and lead. For example, Japanese Patent Publication No. 52-30473(1977) suggests Rh-based intermetallic compounds comprising Rh and at least one element selected from a group of Sm, Gd, Tb and Dy and/or a group of Ho, Er, Tm and Yb such as GdRh and Gd.sub.0.5 Er.sub.0.5 Rh Japanese Laid Open Patent Publication No. 61-86420(1986) describes magnetic substances comprising Er, Al and O in specific content ratios. Japanese Laid Open Patent Publication No. 1-310269(1989) describes a heat accumulator in which alloys of a wide-ranging composition represented by the formula AM.sub.z are used wherein A is a lanthanoid except Lu, M is Ni, Co and/or Cu and z is not less than 0.001 and not more than 9.0.
These alloys have a local maximum of volume specific heat at a temperature of not higher than 30 K. which results from a large entropy increase/decrease caused by the order-disorder transition of the spin system which occurs at a temperature of not higher than 30 K. However, as is described in these specifications, the peak value and the peak temperature widely changes depending on the composition of the alloy. Furthermore, there is no example where the liquid helium temperature is achieved by using these alloys as heat reserving materials.
We studied magnetic properties of superconductive materials and found that R.sub.3 Ru (wherein R represents rare earth metals), which has not yet been studied as a rare earth metal heat reserving material, has excellent properties as cryogenic heat reserving materials and our further study on Ru alloys revealed that intermetallic compounds represented by the formulae R.sub.5/2 Ru and R.sub.5/3 Ru, mixture thereof as well as Ru-based alloys having a similar composition also have excellent properties as cryogenic heat reserving materials. (The composition of these compounds has not yet been strictly confirmed. We use R.sub.5/2 Ru, R.sub.5/3 Ru and the like as representative expressions of the intermetallic compound which has a composition substantially identical or close to the composition represented by these formulae. For example, R.sub.44/25 Ru and R.sub.73/27 Ru are also represented by these formulae in this specification.)